An antigen is any molecule that stimulates an immune response. Most antigens are proteins or polysaccharides, though small molecules coupled to carrier proteins (haptens) can also be antigenic. The segment of an antigenic molecule to which its cognateantibody binds is termed an epitope or antigenic determinant. Immune responses ideally distinguish between self and other. Anergy toward self-targets operates as one self-tolerance mechanism to control the autoreactive cells found in disease-causing autoimmunity.

Immature, precursor dendritic cells (pDC) circulate throughout the body, migrating to lymphocyte rich tissues (such as spleen and lymph nodes) upon stimulating encounter with antigen. The dendritic cells internalize the antigen then externalize (fragmented) antigen that they present to lymphocytes in MHC-peptide complexes, expressing markers that stimulate lymphocyte activation.

Monocyte → macrophage activationProduction of the macrophage lineage from progenitors in the bone marrow is typically controlled by M-CSF, which is constitutively expressed by many cell types. Serum levels of M-CSF and GM-CSF increase in response to invasive stimuli and inflammation, and monocyte numbers increase dramatically. M-CSF-derived macrophages are larger, and have a higher phagocytic capacity, while GM-CSF-derived macrophages are more cytotoxic against TNF-α-resistant tumour targets, express more MHC class II antigen, and constitutively secrete more PGE-2.

Activation of the resting Tc cell involves two steps: 1) TCR on the CD8+ cell interacts with antigen-class IMHC complex on the surface of a target cell. 2) CD8+ Tc cell is stimulated by cytokines, particularly IL-2, which have been secreted predominantly by activated Th cells. Resting Tc do not express IL-2 receptors until antigen stimulation increases the expression of Tc IL-2 receptors, ensuring that activation is confined to Tc cells that ligate cognate antigen. Activated Tc cells become CTLs.

The first signal for helper T cell(Th) activation is interaction of the TcR-CD3 complex with antigen-MHC class II molecules on the surface of an antigen presenting cell. Stimulation is aided by the CD4 molecule on Th cells, with or without assistance from other accessory molecules, such as CD45, CD28 and CD2. Increased IL-2 secretion by the T cell and an increase in IL-2 receptors on the T cell surface trigger a cascade of biochemical events.

The mechanisms by which affinity maturation is achieved are somatic hypermutation and clonal selection. Somatic hypermutation (SHM) is a diversity generating, regulated cellular mechanism through which antibodies are produced against an enormous variety of different potential antigens. The binding affinities of the variable regions of immunoglobulins are altered by AID-enzyme-promoted mutations during antigen-stimulated proliferation of B cells. These somatic hypermutations are transcribed and translated into thousands of slightly different immunoglobulins coded by the hypermutated V regions. The complementarity determining regions of these antibodies possess different affinities for the encountered antigen, and clonal selection will favor cells equipped with highest affinity antibodies because these B cells are favoured in terms of activation and co-operation with T cells.

Clonal selection is the phenomenon whereby a previously unencountered cognateantigen (epitope) can stimulate naïve Blymphocytes to proliferate and differentiate into clones of memory B cells and plasma cells that produce antibodies with the highest affinity for the antigen. Those B cells that have highest affinity BCR against the encountered antigen will be selected for proliferation, antibody production, and committment to an antigen-specific memory lineage.

Thus, SHM prepares a spectrum of antibodies with different affinities for the antigen, while clonal selection ensures that the immune system will react increasingly effectively (highest affinity) to an encountered antigen and will be ready for rapid response to subsequent encounters with the antigen.

Anergy (immunologic tolerance) refers to the failure to mount a full immune response against a target.

Anergy toward self-targets operates as one self-tolerance mechanism to control the autoreactive cells found in autoimmunity. Clonal deletion in which lymphocytes are killed if they recognize a self-antigen during their maturation in the thymus gland or bone marrow is a major mechanism for the prevention of autoimmunity. However, not all human self-antigens are expressed in the central lymphoid organs where the lymphocytes are developing. Thus, self-tolerance to an individual's own antigens must also depend on mechanisms such as clonal anergy. Theoretically, recognition of a self-antigen eliminates the proliferative capacity of autoreactive lymphocytes in the peripheral immune system. Another process, immunoregulation, utilizes regulatory T cells that weaken harmful or inappropriate lymphocyte responses.

In B cell anergy, self-reactive B cells persist in the periphery yet remain unresponsive to immunogen. Research findings indicate that continuous binding of antigen and subsequent receptor signaling are essential for the maintenance of anergy.[n]

During a productive immune response, CD4+ T cells respond to effective signals by producing interleukin 2 (IL-2) and by proliferating. Effective signals stimulate require both ligation of TCRs with cognateantigens presented by class II MHC molecules on the surface of APCsand activation of costimulatory receptors, such as CD28, which recognize ligands such as B7 proteins expressed on the surface of APCs.

When T cells receive stimulus onlyTCR signals in the absence of engagement of costimulatoryreceptors, they enter a state of anergic unresponsiveness characterized by an inability to produce IL-2 or to proliferate upon re-stimulation. Such anergic T cells show a profound block in Ras/MAPK pathway that prevents activation of the AP-1 family of transcription factors (Fos/Jun).

GRAIL (gene related to anergy in lymphocytes) is GRAIL is an E3 ubiquitin ligase that is necessary for the induction of CD4+ T cell anergy in vivo. It is upregulated in naturally occurring (thymically derived) CD4+ and CD25+ cells [a] and anergized T cells [1]. Both GRAIL and Foxp3 are genotypic marker for CD25+ Treg cells. T cell activation appears to be controlled by Foxp3 through transcriptional regulation of early growth response (Egr) genes Egr-2 and Egr-3, and E3 ubiquitin (Ub) ligase genes Cblb [?], Itch [?] and GRAIL, subsequently affecting degradation of two key signaling proteins, PLCgamma1 and PKC-theta. [a]

It is believed that GRAIL could induce anergy through ubiquitylation of membrane-associated targets required for T-cell activation. It has been demonstrated that two isoforms of otubain-1, in conjunction with the deubiquitylating enzyme USP8, produce opposing effects on the expression and function of GRAIL in the induction of anergy.[2] GRAIL is differentially expressed in naturally occurring and peripherally induced CD25+ Treg cells where the expression of GRAIL has been suggested is linked to their functional "regulatory" activity.

Immunoglobulins (left - click to enlarge) comprise two heavy (h) and two light-chain (l) protein subunits, each of which folds into domains (4 on heavy, 2 on light). These adhesion sites or domains contain one or more folds of 60 to 100 amino acids.

Depending upon the character of the heavy chain, immunoglobulins are divided into five classes – IgG, IgD, IgE, IgA, IgM – that are expressed in different tissues. The classes are further subdivided into isotypes, which have different properties in terms of complement fixation and binding to immunoglobulin (Ig) receptors.

Membrane-bound Igs have a transmembrane segment and a cytoplasmic C-terminal tail. The 2 β- chains are stabilized into sandwiched β sheets that are adherent by virtue of hydrophobic interactions between disulphide bonds. Igs assume a Y-shaped structure "topped" at the extracellular N-terminals by variable domains (red), with a variable domain at the tip of the heavy chain (1) and the light chain (2), between which lies an antigen binding site (3). The variable regions are coded by pluripotential DNA sequences that can generate thousands of polypeptide sequences capable of adhering to millions of different ligands. Binding is homophilic or heterophilic, including binding to different Igs and to integrins. Both light and heavy chains contain constant domains (white, 4).

Right - click to enlarge - the heavy chains of IgA, IgD and IgG each have four domains, where those at the N-terminal are variable (VH) and the other three are constant (CH1-3). IgE and IgM have one variable and four constant domains (CH1-4) on the heavy chain. The variable domains are termed Fab, while the constant domains are termed Fc.

The light chains have two domains, one variable domain (VL) at the N-terminal, and one constant (CL) domain.

The antigen binding site lies between VH and VL (shaded lavendar). Most variability is found in three superficial-loop forming regions in the VH and VL domains, which are the complementarity determining regions or CDRs. CDR3 binds antigens and CDR1-2 bind MHCs. CDR3 shows more variation that do either CDR1 or 2.

The domains have related amino acid sequences that possess a common secondary and tertiary structure. This conserved structure is found frequently in proteins involved in cell-cell interactions and is particularly important in immunology. The constant (Fc) regions have complement fixing and Ig receptor binding activity. The hinge region, in IgG, IgA and IgD, is an important sequence of 10-60 amino acids between CH1 and CH2 that confers flexibility on the molecule.

Immunoglobulins attain their enormous variability by splicing components (VDJ recombination) coded in widely scattered sequences of DNA that are located in two different chromosomes. Antigen binding takes place at the heavy chain, which displays enormous variation by virtue of combining 1 of 400 possible variable gene segments with 1 out of 15 diversity segments and 1 out of 4 joining segments. This alternative splicing generates 24,000 possible combinations for the DNA encoding the heavy chain alone. The variable coding segments are assembled together with those for the constant-C segments of the heavy-chain molecule.

Some antibody classes form multimeric structures – pentamers (IgM) and dimers or trimers (IgA). These two isotypes also associate with a small protein called the joining (J) chain required for stabilisation of the complexes.

The immunoglobulin superfamily is evolutionarily ancient, is widely expressed, and is constitutive or long-term up-regulated. Immunoglobulin antibodies are released by activated B cells of the immune system, on which they also act as surface marker proteins. Adherence of immunoglobilins to foreign substances or to cellular invaders may be sufficient to disarm the invader, or the attached antibodies function as attack signal to macrophages and natural killer cells. Adhesion molecules of the immunoglobulin supergene family, activate specific kinases through phosphorylation, resulting in activation of transcription factors, increased cytokine production, increased cell membrane protein expression, production of reactive oxygen species, and cell proliferation.

An antigen is any molecule that stimulates an immune response. Most antigens are proteins or polysaccharides, though small molecules coupled to carrier proteins (haptens) can also be antigenic. The segment of an antigenic molecule to which its cognateantibody binds is termed an epitope or antigenic determinant.

Antigens are classified by immune activity as immunogens, tolerogens, or allergens according towhether the molecule in question activates the immune response, is tolerated by the immune system, or elicits an allergic response, respectively. Allergic reactions are exaggerated immune responses to molecules (allergens) that would otherwise not prove harmful. Antigens may also be classified according to their source as exogenous, endogenous, autoantigenic, or tumor antigens.

Endogenous antigens are internally generated molecules that become presented on the cell surface in the complex with class I histocompatibility molecules (MHC I). Endogenous antigens may result from exogeneous viral or bacterial infections that have altered the host cell.

Tumor-specific antigens (TSAs) typically result from a tumor specific mutation and are targetted for non-self attack when displayed on class I histocompatibility molecules. Tumor-associated antigens (TAAs) are more common than TSAs, and are presented both by tumor cells and by normal cells. Tumor antigens may elicit targetting by CTLs before the tumor cells can successfully proliferate and metastasize. Unfortunately, tumors employ a variety of mechanisms to evade the immune system.

Protein antigens are T dependent in that they require T cellco-operation to induce antibody responses in B cells. Non-protein antigens, such as polysaccharides and lipids can elicit T-independent antibody responses. Such T-independent antigens are typically polymeric, so it is believed that they are able to cross-link BCR-surface-Ig sufficiently strongly to activate B cells without T cell costimulation. These T-independent polymeric antigens elicit IgMantibodies and do not demonstrate affinity maturation. However, a subclass of T cells are specialized to presentlipid and glycolipid antigens – γδ T cells recognize foreign nonpeptide antigens presented by CD1 proteins, which are MHC-like-molecules specialized for the presentation of lipids.